The ISO 6743 standard “Classification of lubricants, industrial lubricants and related products (Class L)” divides lubricant products into 18 groups, which are arranged according to the letters A to Z. The lubricants are classified into 18 groups.
A：Total loss systems
B：Mould release
C：Gears
D：Compressors (including refrigeration and vacuum pumps)
E：Internal combustion engine
F：Spindle bearings,bearings and associated clutches
G：Slideways
H：Hydraulic systems
M：Metal working
N：Electrical insulation
P：Pneumatic tools
Q：Heat transfer
R：Temporary protection against corrosion
S：Applications of particular lubricants
T：Turbines
U：Heart treatment
X：Applications requiring grease
Y：Other applications
Z：Steam cylinders
Solid lubricant Although the history of this type of lubricant material is not long, but its economic effect is good, wide range of adaptability, rapid development, can adapt to high temperature, high pressure, low speed, high vacuum, strong radiation and other special conditions of use, especially suitable for the inconvenience of the oil supply, installation and dismantling of the difficult occasions. Of course, it also has the disadvantages of higher friction coefficient and poor cooling heat dissipation. It is customary to classify solid lubricants into two categories: inorganic and organic. The former includes graphite, molybdenum disulfide, oxides, fluorides, soft metals and others; the latter includes PTFE, nylon, polyethylene, polyimide and so on. According to its shape can be divided into solid powder, film and self-lubricating coating materials.
Solid powder can be dispersed in gases, liquids and colloids; film has spray coating, vacuum deposition, flame spraying, ion spraying, electrodeposition, sintering and so on; as for the production process of laminating materials is more diverse, is an emerging important lubricating materials. Graphite has natural graphite and artificial graphite, it is a hexagonal crystal layer structure. Graphite is a black, soft, chemically very stable material, almost not subject to all organic solvents, corrosive chemicals erosion, but also not subject to many of the characteristics of molten metal or molten glass immersion. Therefore, graphite does not lose its characteristics when mixed with water, solvents, grease, rubber, resin and some metals. Graphite’s coefficient of thermal expansion and modulus of elasticity are small, and can resist thermal shock.
Graphite has a temperature range of -270-1000℃ (in atmosphere), melting point of 3500℃, oxidized at 450-500℃. In addition, graphite has good electrical and thermal conductivity. Graphite’s crystallinity, impurities, particle size and particle shape have a greater impact on graphite’s lubricity, and external conditions of use such as ambient temperature, speed of use temperature and load also have an impact on its lubricity. The application of graphite lubricant is mostly seen in the laminating materials or shared with other solid lubricants, and there are not many lubricants using graphite alone, such as water-based graphite lubricant, oil-based graphite lubricant, and so on.
Molybdenum bisulfide has a black-gray metallic luster, the same hexagonal crystal layer structure, the friction coefficient can be as low as 0.04, and are more stable to heat or chemical. The use of molybdenum disulfide temperature: -270-350 ℃ (atmospheric), melting point of 1250 ℃, 380-450 ℃ when oxidized. Molybdenum disulfide can resist corrosion of most acids. Oxidation of molybdenum disulfide in room temperature, wet air is slight, but the result can get a considerable acid value. It is generally believed that for heavy loads, low and medium speeds, high (low) temperature sliding friction parts, the application of molybdenum bisulfide powder can give play to its excellent results, commercially available molybdenum bisulfide powder purity of 98-99.8%. Rarely used molybdenum disulfide monomer powder, in most cases often mixed with other substances necessary. Commonly available are molybdenum disulfide paste lubricants and molybdenum disulfide lubricating grease.
Plastics such as PTFE have good lubrication, shock absorption, impact resistance, corrosion resistance and insulation. The operating temperature of PTFE is: -270-260℃.
Soft metals such as gold, silver, zinc, lead, and tin are used as solid lubricants in two ways: one is in the form of a thin film, such as lead, zinc, and tin, which are low-melting metals, and copper and bronze, which are not low-melting and are sometimes used in this way.
Liquid Lubricants This is the largest and most diverse class of lubricants, including mineral oils, synthetic oils, animal and vegetable oils, and water-based liquids. Because these fluid lubricants have a wide range of viscosities, they offer a wide range of options for moving parts operating under different loads, speeds and temperatures. Fluid lubricants can provide low and stable friction coefficients, low compressibility, can effectively carry away heat from the friction surfaces, to ensure the dimensional stability of the relative moving parts and equipment accuracy, and most of them are inexpensive products, and thus widely used. Mineral oil is currently the largest amount of a liquid lubricant. Water has good thermal conductivity, abundant resources, inexpensive and easy to obtain, but its viscosity is too small, so it is necessary to add viscosity enhancers or oily agents. Currently used in large quantities of water-based cutting fluid and water-glycol hydraulic fluid, which is a very promising class of lubricant materials, assuming that in the future, when the world’s petroleum resources are depleted, this will be an important lubricant material instead of mineral oil. Animals and vegetable oils are mainly vegetable oils, such as rapeseed oil, tea seed oil, castor oil, peanut oil and sunflower oil, etc. The advantages are good oleophilic properties and good biodegradability. The disadvantages are poor oxidative and thermal stability, and the low-temperature performance is not good enough. At present, it is still an important component of some cutting fluids. With the gradual shortage of petroleum resources and increasingly demanding environmental requirements, people are re-emphasizing the development and application of animal and vegetable fats and oils as lubricating materials, hoping to improve their thermal and oxidative stability and low-temperature performance through chemical methods, as an important lubricating material in the future instead of mineral oil. Synthetic oils were developed during the Second World War. Synthetic oils contain many different types of compounds with different chemical structures and properties, and are mostly used under relatively harsh working conditions. First used in the military, gradually promoted to civilian use, which is an important lubricant material to replace mineral oil in the future. In recent years, synthetic lubricating grease has been more widely used and recognized.
Gas Lubricants Gas can be used as a lubricant in the same way as oil, applying the same laws of physics that apply to fluid power lubrication, but can also be applied to gases. The low viscosity of gases means that their film thickness is also very thin. Therefore, fluid-dynamic gas bearings (gas-dynamic bearings) are only suitable for high speeds, light loads, small clearances and very tight tolerances. For this reason, the more commonly used pneumatic bearings can withstand higher loads, have less stringent clearance and tolerance requirements, and can be used at lower speeds, even at zero speed.
Gas lubrication can be used at higher or lower temperatures than lubricating oils and greases, and can be used to lubricate sliding bearings in the range of -200°C to +2000°C, with an undetectable coefficient of friction and high bearing stability. High rigidity can be obtained in high-speed precision bearings (e.g., medical drills and spindles for precision grinding machines and inertial navigation gyroscopes, etc.), and there are no sealing and contamination problems. The disadvantage is the low load carrying capacity, which requires a high degree of difficulty in the design and machining of the bearings. The bearing surface is easily damaged during driving and stopping. Commonly used gas lubricants are air, helium, nitrogen and hydrogen. Requirements for cleanliness is very high, must be strictly refined before use. Semi-solid lubricants which are semi-fluid at room temperature and pressure, and have a colloidal structure of the lubricant material, called grease. Generally divided into soap-based grease, hydrocarbon-based grease, inorganic grease, organic grease four. In addition to their anti-moisture, wear-reducing properties, but also sealing, vibration damping and other functions, and make the lubrication system is simple, easy to maintain and manage, saving operating costs, and thus gained widespread use. Its disadvantage is that the mobility is small, poor heat dissipation, high temperature is prone to phase change, decomposition and so on. Lubricating grease production accounted for the proportion of the total output of the entire lubricant is not large, about 2% or so, but in the field of lubrication plays a large role, according to statistics, about 90% of rolling bearings are lubricated with grease. According to statistics, about 90% of rolling bearings are lubricated with grease. Moreover, about 43% of the current failures of rolling bearings are caused by improper lubrication. Therefore, it is important to pay attention to the quality and product composition of grease. Lithium-based greases are widely used because of their excellent performance in various aspects. In advanced industrialized countries, lithium-based grease accounts for more than 60% of their total grease production.Production of lithium-based grease reached over 60% for the first time in 2000.
Comparison of four types of lubricants
1、Fluid power lubrication properties: oil excellent, grease average, solid lubricant none, gas good
2、Boundary lubrication properties, oil poor to excellent, grease good to excellent, solid lubricant good to excellent, gases poor
3、Cooling performance: oil very good, grease poor, solid lubricant no, gas general
4、Low friction: oil fair to good, grease fair, solid lubricant poor, gases excellent
5、Easy to add to bearings: oil good, grease average, solid lubricant poor, gas good
6、Ability to remain in the bearing: oil poor, grease good, solid lubricant very good, gas very good
7、Sealing ability: oil poor, grease very good, solid lubricant fair to good, gas very good
8、Corrosion protection against atmospheric corrosion: oil fair to excellent, grease good to excellent, solid lubricant poor to fair, gases poor;
9、Temperature range: oil fair to good, grease good, solid lubricant very good, gases excellent
10、evaporation: oils very high to low, fats usually low, solid lubricants low, gases very high
11、Flash: oils very high to very low, fats usually low, solid lubricants usually low, gases depending on gas type
12、Compatibility: oil very high to average, grease average, solid lubricants excellent, gases usually good
13、Lubricant prices: oil low to high, grease quite high, solid lubricants quite high, gases usually very low
14、Bearing design impurities: oil very low, grease very low, solid lubricants low to high, gases very high
15、Life is determined by: oil degradation and contamination, grease degradation, solid lubricant wear, and the ability of the gas to maintain gas resupply.
A solution to be considered when pure mineral oil does not meet the requirements of the bearing:
1、Too much load: Use more viscous oil, extreme pressure oil, grease, solid lubricant.
2、Speed too high (may cause high temperatures): Increase lubricant volume or oil circulation, less viscous oil, gas lubrication
3、Temperature too high: use additives or synthetic oils, more viscous oils, increased oil volume or oil circulation, solid lubricants
4、Temperatures too low: lower viscosity oils, synthetic oils, solid lubricants, gas lubricants
5、Too much wear debris: Increase oil volume or oil circulation.
6、Contamination: oil circulation systems, grease, solid lubricants
7、Longer life required: Viscous oils, additives or synthetic oils, heavier oils, or oil-circulating greases.
The two main factors that influence the choice of lubricant type are speed and load.
